The present invention relates to a conically scanning antenna system for tracking radars.
A tracking radar measures the co-ordinates of a target and provides data which can be used to determine the trajectory of the target and to predict its future position. To make such a prediction, a wide variety of data available from a radar can be used such as range, elevation angle, azimuth, or Doppler frequency. This means that any radar may prima facie be considered a tracking radar as soon as the output information which it provides is processed in a suitable manner. However, a tracking radar is distinguished from other radars by the way in which the angular tracking of the target is performed, and the object of this angular tracking is to define an error which indicates the angular divergence between the axis of the antenna (known as the boresight axis) and the direction in which the target lies, this error signal being fed to servo-mechanisms designed to realign the antenna axis with the direction of the target. Among methods which have become conventional for producing such an error signal, we may mention sequential lobing, conical scanning, and the monopulse method.
The antenna system according to our invention relies on the second method, i.e. that of conical scanning, whose principles will now be reviewed. In a conical-scanning system, the antenna is provided with a centrally symmetrical paraboloidal or reflector lens which is illuminated by a primary feed whose phase center describes about the boresight axis of the system a circle of predetermined radius lying in the focal plane. In such an antenna, the radiation pattern is no longer centered on the boresight axis but rotates in space in such a way that the direction of maximum radiation traces out a cone whose half apex angle is termed the squint angle of the antenna.
The amplitude of the signal provided by the antenna is thus modulated at the frequency of rotation of the radiation pattern and the depth of modulation is a function of the angle of the target relative to the axis of rotation. The modulation signal extracted from the echo signal is used in servomechanisms to slave the position of the antenna to the target.
Because of the central symmetry of the focusing system, the beams radiated by the antenna all overlap on the boresight axis and in general the level of overlap is such as to be of an optimum value which represents a compromise between the initial inclination, determining the aiming accuracy, and the range of the radar.
In a conventional, conically scanning antenna the radiation pattern is the same at transmission and reception and this provides an opportunity, by analyzing the transmission pattern, of finding the frequency of rotation of the pattern, which can be utilized for interference purposes.
There are applications where it must be made impossible for the frequency of rotation of the radiation pattern of a conically scanning antenna to be detected in this way.
It has already been proposed to transmit outgoing waves with a radiation pattern centered on the axis of the antenna and to receive incoming waves, reflected by outlying targets, with a conically scanning radiation pattern. An arrangement based on this principle has a primary feed of the monopulse type which feeds signals to a sum channel and two difference channels, one for elevation and the other for azimuth. The sum channel is combined with the difference channels and the conical scanning pattern is obtained, at reception, by means of a rotating variable phase shifter, which causes a phase variation between the sum and difference signals. The radiation pattern so obtained is eccentric and rotates at the speed of the phase shifter. This system provides a single-channel receiver which is not, however, proof against errors in determining angles due to fluctuations in the amplitude of the echo. What is more, the resulting equipment is relatively complicated and thus expensive.